In a typical paper making process, logs are debarked and chipped, and individual cellulose fibers are then freed or liberated from the chip for subsequent treatment and ultimate paper web formation. A common way to liberate the cellulose fibers is by cooking the wood chips with chemicals at elevated temperatures and pressures in digesters to remove lignen from the chips, which holds the fibers together. For the subsequent paper making process, it is desirable that the delignified fibers obtained exhibit substantially similar characteristics. To minimize the production of undercooked or overcooked chips in the digester, it is necessary that the cooking liquor penetration into the chips is substantially similar for all chips, so that the effects of temperature, pressure, and time are similar for all chips. Therefore, it has been found desirable in the past to utilize chip screening apparatus which removes both undersized and oversized chips, so that the undersized can be treated separately and the oversized passed through chip size reducing apparatus prior to digesting.
A commonly used apparatus for reducing the size of oversized chips separated from a chip stream by screens is a chip slicer. The basic operation of a chip slicer includes a rotor operating within a drum, wherein the oversized chips are forced against knives and are thereby sliced to acceptable thickness. An example of a chip slicer can be found in U.S. Pat. No. 4,235,382 issued to William C. Smith for a "Method and Apparatus for Rechipping Wood Chips". While chip slicers such as that taught in U.S. Pat. No. 4,235,382 work effectively to reduce the size of oversized chips, thereby substantially reducing the occurrence of undercooked chips in a digesting process, chip slicers which are not working within optimum design parameters, such as when knives are dull, or improper speed or loading occurs, tend to generate fines while reducing oversized chips. Thus, while minimizing the problem associated with oversized chips, chip slicers tend to increase the problem of undersized chips or fines. Therefore, it is desirable to develop an apparatus for treating oversized wood chips which does not compound the problems associated with fines or undersized chips.
Closely operating rolls have been utilized in the past for treating oversized chips by compression, and thereby affecting liquor penetration into the chips. For example, U.S. Pat. No. 4,050,980 issued Sept. 27, 1977 to Fred L. Schmidt and Frank J. Steffes for "Selective Delamination of Wood Chips". This patent teaches screening a chip stream and passing the oversized chips through closely operating rolls for selective delamination by compression.
U.S Pat. 3,393,634 issued July 23, 1968 to John M. Blackford for a "Method and Apparatus for Loosening Fibers of Wood Chips". This patent teaches closely operating rolls with an apparatus for directing chips edgewise into the crotch between the rolls, with the rolls compressing the chips transversely of their thickness to at least about one-fifth of their original thickness, but not more than about one-tenth of their original thickness. Thereafter, the chips are allowed to expand to their original shape, with the fibers therein having been loosened and the porosity of the chips having been increased.
In each of the two above-mentioned patents, the opposed, closely operating rolls, or delamination rolls compress the chips for loosening the fibers therein. The rolls are smooth, so that the only action on the chips is compressive, whereby the chip structure is not substantially changed other than for a loosening of the fibers.
A problem associated with the use of delamination rolls is that throughput is low. Chips tend to stay in the pocket above the rolls, and, particularly the larger chips which are most in need of delamination, tend to ride between the rolls in the upper portion of the roll couple, without being drawn through the rolls.
A typical structure for a chip destructuring apparatus is disclosed in an article entitled "A Machine For Destructuring Wood Chips by Rolling" by John A. Oldham in the July 1983 issue of APPITA, Volume 37, Number 1. In the last paragraph of the first column on Page 65, the destructuring machine is described as having "smooth, chrome surfaced, very rigid rollers". The aforedescribed problem of passing larger chips through the nip is discussed in the first paragraph on Page 66. The larger chips "often would not enter between the smooth rollers; the surface of the rollers slipped over the chips". It is then described that the chips remaining above the rolls obstructed feeding of succeeding chips causing clotting or bridging. In the third paragraph on Page 66, a solution is discussed wherein small grooves, only one millimeter deep were cut parallel to the roll axis at approximately 10 millimeter spacings. Harsher roll surfaces are not deemed appropriate, since an unacceptable amount of fiber damage would be created. General roughening of the roll surface is also described as being likely to improve feed reliability.
An analysis of the effects of chip destructuring or delamination was presented at the 1984 TAPPI Pulping Conference by D. Lachenal, P. Monzie, and C. deChoudens in a paper entitled "Chip Destructuring Improves Kraft Pulping", TAPPI November 1984, Book 1, Pages 13-17. In the apparatus used for the pulping trials discussed in the article, again the rollers were smooth, and the chips were compressed.
Destructuring or delamination as known previously has not been accepted as a standard process in pulping operations, largely, it is believed, due to the low capacities of delamination devices and inconsistent results and subsequent effects on digesting operations.
It is therefore one of the principal objects of the present invention to provide an apparatus for treating oversized chips in a manner to reduce the necessary cooking time therefore, to achieve in the treated oversize chips delignification levels similar to that for smaller chips during identical delignification processes, with resultant pulps having similar characteristics and properties.
It is another object of the present invention to provide an apparatus for treating oversized chips quickly and efficiently with rapid throughput, while minimizing plugging or blinding of the apparatus.
It is yet another object of the present invention to provide a wood chip treating apparatus which cracks or fractures oversized chips without generating additional fines or pin chips, and which is simple in operation, requiring minimal adjustment for optimal operation.
A still further object of the present invention is to provide an apparatus for treating wood chips to increase the rate of liquor impregnation particularly of large chips and for providing an apparatus to destructure wood chips which is not dependent on a particular chip orientation between the closely operating rolls.